1. Field of the Invention
This invention relates to the discovery of certain critical operating parameters for an airless, pulsing spray dampening system suitable for use either on new offset lithographic presses or as a retrofit on older presses. Utilization of the parameters disclosed herein enables simplified yet accurate control of the ratio of ink to dampening fluid applied to the plate cylinder such that dampening fluid is evenly distributed over the entire area of the plate cylinder and the likelihood of excessive emulsification of the ink is substantially reduced.
2. Description of the Prior Art
The printing plate of an offset lithographic press typically has areas that have been previously etched for reception of an ink solution comprised of vehicles, pigments, dryers and other miscellaneous ingredients. The remaining, unetched portions of the plate comprise the non-image areas that are commonly supplied with dampening fluid or so-called "water" in order to resist the deposition of ink in these areas. As is known, high quality printing can be obtained only when the correct ink/dampening fluid balance is maintained. However, the ratio of ink to dampening fluid is a function of the type of plate, the number of image areas on the plate, the composition of the paper and also the chemistry of the dampening solution.
Failure to maintain the correct ink/dampening fluid balance can lead to disastrous results within a short period of time. Excessive emulsification of the ink occurs when an oversupply of water flows to the plate, and can dilute the strength of the ink and cause the printed image to lack sharp contrast. Additionally, excessive emulsification causes the ink to rub off the paper and onto the reader's hands. Oftentimes, it is necessary to stop the press once such excessive emulsification is observed, so that the ink train may be completely cleaned before a large quantity of printed copy is adversely affected.
Various means are available for dampening the lithographic plate. In some presses, a dampening roller rotates within an open trough containing a quantity of the dampening solution, and this roller in turn rotates against a series of transfer rollers associated with the plate cylinder to supply a film of fluid to the plate. Unfortunately, such systems are difficult to precisely control and the fluid is subject to evaporation and contamination.
In recent years, the use of spray dampening systems has become increasingly popular. Such spray dampening systems are disclosed, for example, in U.S. Pat. Nos. 3,651,756 to Smith, Jr., dated Mar. 28, 1972, as well as 4,064,801 to Switall, et al., dated Dec. 27, 1977. Both of these patents describe spray dampening systems wherein a valve means repetitively opens and closes to create a pulsed stream of water which is then directed through nozzles toward a dampening roller associated with the plate cylinder. The amount of water sprayed toward the dampening roller is automatically varied in accordance with the speed of the press.
Another type of dampening apparatus is illustrated in U.S. Pat. No. 4,469,024 dated Sept. 4, 1984 and concerns a pulsed spray dampener wherein an air pulse from a unit controller mechanically operates four air actuated valves which in turn interrupt a flow of dampening fluid through four nozzles respectively associated with the valves. However, the spray from the nozzles shown in U.S. Pat. No. 4,469,024 is directed toward a nip between an ink drum and an adjacent ink form in rolling contact with both the ink drum and an adjacent plate cylinder, which therey causes dampening fluid to become undesirably mixed with the ink and adversely change the quality of the printed image. Also, the response time normally associated with air actuated valves is relatively slow and somewhat imprecise and as a result the characteristics of the pulse of dampening fluid leaving the spray nozzle, such as the pulse duration, timing and frequency, cannot be precisely controlled within satisfactory tolerances.
When constructing a spray dampening system, it has been common practice to provide four nozzles spaced across the width of a press handling a double width newspaper such that each nozzle directs the spray toward a corresponding portion of the adjacent dampening roller. Typically, the double width of newspaper corresponding to four printed pages has a dimension ranging from 55" to 63", although in the United States, paper widths are commonly 55". As a result, each of the spray nozzles is positioned to provide coverage for approximately 27.5" to 31.5" of the adjacent dampening roller, and usually each nozzle is spaced 8" or more away from the roller so that the diverging spray has sufficient distance to spread. Unfortunately, existing presses often have insufficient clearance for mounting a retrofit spray system in the aisle between the presses at a location 8" from a dampening roller, and consequently such system must be installed within the arch of the press where acess is somewhat difficult.
Moreover, the pressure of the dampening solution applied to the nozzles of a spray system must be retained at relatively high levels in order to ensure that the spray is not only dispersed laterally across a sufficient width of the roller but also propelled with substantial forward velocity to ensure contact with the roller. Oftentimes, such pressures are maintained at 80 psi resulting in increased equipment as well as operating costs. The location of the nozzles at a distance of 8" or more spaced from an associated roller is also believed to impede any effort to effect an even dispersal of very small dampening fluid droplets, since the relatively large distance from the nozzle to the roller allows droplets to coalesce to form larger drops and consequently cause the spray to miss certain portions of the roller.
Additionally, spray dampeners often have nozzles that are coupled to a source of pressurized air and are arranged so that the pressure of the air assists in transporting the dispersed spray toward the roller. Unfortunately, it has been found that the air passageways often clog due to the large amount of airborne paper particulates and dust in the environment surrounding the press, even through the air itself may have been previously filtered. As can be appreciated, as soon as the nozzles are clogged, operation of the press must be interrupted to restore communication between the various passageways.
It is known that prior art spray dampeners cause the plate to experience moisture variations cycling between wet and dry conditions. Because ink has the ability to "hold" water, the user often increases the flow of dampening fluid to the plate in an attempt to create a water reservoir and avoid intermittent dryness. However, this practice results in such excessive plate moisture that the problems of unstable emulsification and ink rub-off soon become apparent.
In some instances, dampening systems have a roller in contact with the plate, and the roller is provided with an outer sleeve comprising a relatively soft cloth material. During operation of the press, dampening fluid soaks into the cloth and the sleeve is thereafter operable as a reservoir for the fluid, such as is particularly desirable where a ductor roller is used to intermittently supply dampening fluid from a water pan to the cloth covered roller. However, the use of a cloth sleeve prevents the nip between the cloth covered roller and the adjacent plate cylinder from uniformly smoothing the dampening fluid over the entire area of the roller as well as the outer surface of the plate cylinder. Unless the dampening fluid is evenly distributed to the plate, however, water spots will become apparent on the printed image, and such accumulations of water are generally difficult to quickly remove from the rollers of the press and, at the same time, a large quantity of paper from such press runs may have to be discarded because of the resulting poor quality image. Water spotting is particularly a problem at high press speeds and when relatively fine quality printing is sought. The paper sleeve is also known to cause slippage between the covered roller and any adjacent rollers and this slippage, as can be understood, further hinders the uniform distribution of dampening fluid across the rollers.
More importantly, it is desirable to provide a dampening control system that requires the least amount of operator attention and intervention. Unfortunately, prior art dampening systems with nozzles spaced 8" from the rollers and spraying approximately 120 pulses of dampening fluid per minute often cause an improper balance of the ratio between the dampening fluid and the ink, such that the operator must keep a watchful eye over the press at all times. Nonetheless, the ink/dampening fluid balance of such systems frequently becomes incorrect, whether or not due to the fault of the operator, and thereby the quality of the printed image is ruined.
It is oftentimes difficult to effect a uniform distribution of dampening fluid on a roller whenever a pressurized supply of dampening fluid is sprayed through a nozzle toward the roller. A continuous spray is somewhat undesirable because the small volume of dampening fluid that is required by the plate during each rotation of the plate cylinder cannot be uniformly distributed on a continuous basis. Also, it has been found that nozzles are typically spaced apart a distance from each other and are also spaced at distances from the associated roller such that an overlap of spray patterns on the roller resulting from adjacent nozzles causes an uneven distribution of dampening fluid along the length of the roller to a degree that cannot be corrected by subsequently metering the fluid through a nip between rollers.
Accordingly, it would be a desirable advance in the art if a simplified dampening system were constructed that would provide automatic control at all times for the amount of dampening fluid transferred to the plate regardless of press speed. Additionally, such a system would desirably be located adjacent the aisles between the press units so that maintenance of the system, when necessary, is facilitated. At the same time, such a system should operate satisfactorily without the use of pressurized air, so that costs of the system are maintained at relatively low levels.